1. Field of the invention The present invention relates to electron beam generators used in microwave tubes and particle accelerators and more particularly to an electron beam generator able to operate in either a pulse or a continuous mode such that when operating in pulse mode the electrons coming from a cathode are produced only during very short periods of time and the electron beam is broken up.
The invention applies more particularly to microwave tubes with longitudinal interaction, such as progressive wave tubes or klystrons.
2. Description of the Prior Art
An electron beam is generated by an electron gun which is often built around an axis of revolution. An electron gun comprises mainly a thermionic cathode, heated by a filament and connected to high negative voltage. The cathode emits a beam of electrons towards an anode with an aperture in its center to let the electron beam pass through.
Having gone through the anode, the electron beam enters an application device, in the form of a tunnel, which can be the body of a microwave tube. This device is generally earthed or grounded and finishes with a collector. The anode can be set to the same potential as the application device or an intermediate potential between that of the cathode and that of the application device.
Focussing electrodes and grids can be inserted between the cathode and the anode. All the electrodes going from the cathode to the anode constitute the electron gun.
At present, there exist two main methods of obtaining a pulsed electron beam.
The first consists in modulating all or none of the high voltage supply to the cathode.
The second method consists in introducing a modulation grid between the cathode and the anode. This grid is supplied by a relatively low voltage in pulse mode.
Unfortunately, both of these methods have disadvantages.
In the first method a power modulator is introduced between the high voltage source and the cathode. This power modulator produces a square pulse signal. But the rise and fall time of the signal is long, due to the internal impedance of the high voltage source and the high reactances of the electron gun. In addition, a considerable loss of energy appears due to the energy stored in the parasitic reactances of the supply circuit and the gun. Finally, the electrons produced by the cathode have a variable velocity during the rise and fall of the signal, making it difficult to focus the electron beam.
The second method does not involve the same disadvantages, as the high voltage applied to the cathode remains constant.
In this method a modulation grid is placed between the cathode and the anode.
This modulation grid is supplied with pulses by a voltage close to the high voltage supplying the cathode. Very often, a second grid is inserted between the cathode and the modulation grid, these two grids being approximately parallel and their apertures placed opposite each other. The second grid is set to the same potential as the cathode; it is very close to the cathode and can even rest on it. The electron beam obtained after crossing the grids is made up of many elementary beams. If the operation has high average power, it is necessary to use reduced interception grids, so as to limit overheating.
The electron beam generators operating in continuous mode also generally possess at least one grid placed between the cathode and the anode. This grid is supplied by a control voltage which then enables the current of the electron beam to be adjusted.
However, these grids have structures which introduce aberrations in the elementary beams and these converge badly as a whole. These guns with modulation or control grids do not give satisfactory transmission of the electron beam along the application device. A large part of the power cannot be recovered by the application device, and it is dissipated in a useless and even harmful manner in this device.